4. Calculate the \( \int_0^1 (4x^3 -3x^2)dx \) by definition of Riemann sum.
Hint: $\sum_{i=1}^n i^2 = \frac{n(n+1)(2n+1)}{6}$ and $\sum_{i=1}^n i^3 = \left[\frac{n(n+1)}{2}\right]^2$. (10%)